In recent years, a thin-film solar cell produced using a chalcopyrite compound semiconductor has been put into practical use. This thin-film solar cell produced using a compound semiconductor has a base structure in which a Mo electrode layer serving as a positive electrode is formed on a soda-lime glass substrate; a light-absorbing layer made of a CIGS film is formed on the Mo electrode layer; a buffer layer made of ZnS, CdS, or the like is formed on the light-absorbing layer; and a transparent electrode layer serving as a negative electrode is formed on the buffer layer.
As a method for forming a light-absorbing layer, for example, a method of forming a film by using a multi-component deposition method is known. In a light-absorbing layer obtained by using this method, high-energy conversion efficiency is obtained, but deposition is performed from a point source. Thus, when a film is formed on a substrate having a large area, uniformity of distribution in film thickness is easily degraded. For this reason, a method for forming a light-absorbing layer using a sputtering method has been proposed.
As the method for forming a light-absorbing layer using a sputtering method, the selenization method is used. In the selenization, first, an In film is deposited by sputtering using an In target; and a Cu—Ga binary alloy film is deposited by performing sputtering using a Cu—Ga binary alloy sputtering target on the In film. Next, a CIGS film is formed by heat treating the obtained the laminated precursor film, which is made of the In film and the Cu—Ga binary alloy film, in a Se atmosphere.
Furthermore, a technology, in which a laminated precursor film of the Cu—Ga alloy film and the In film is produced by using a sputtering method in order of a Cu—Ga alloy layer having a high content of Ga, a Cu—Ga alloy layer having a low content of Ga, and an In layer from a metal backside electrode layer side, and the produced laminated precursor film is heat treated in a selenium atmosphere and/or a sulfur atmosphere, has been proposed based on the above technologies (PTL 1). With this technology, the concentration gradient of Ga in the thin film light-absorbing layer is gradually (in stages) changed from an interface layer (buffer layer) side to the metal backside electrode layer side, and thereby a thin-film solar cell having a high open circuit voltage is obtained and separation of the thin film light-absorbing layer from other layers is prevented. In this case, it has been proposed that the content of Ga in the Cu—Ga alloy sputtering target be set to 1 to 40 atomic % (PTL 1).
As such a Cu—Ga alloy sputtering target for forming a Cu—Ga alloy layer, a Cu—Ga alloy sintered body sputtering target which is sintered by performing hot pressing on a Cu—Ga powder mixture produced by a water-atomizing device has been proposed (PTL 2). The Cu—Ga alloy sintered body sputtering target is formed of a single composition (uniform composition). Intensity of peaks other than a main peak (γ phase: Cu9Ga4 phase) in a graph obtained by X-ray diffraction of the Cu—Ga alloy is equal to or 5% less than that of the main peak. An average crystal grain size thereof is 5 μm to 30 μm. The oxygen content obtained in the target is 350 ppm to 400 ppm.
On the other hand, in order to improve power generation efficiency of a light-absorbing layer formed from a CIGS film, it is believed that addition of Na to the light-absorbing layer through diffusion from an alkaline glass substrate is effective (NPL 1). However, in a case of a flexible CIGS solar cell using a polymer film or the like as a base instead of the alkaline glass, because the substrate is not an alkaline glass substrate, it is disadvantageous in that a supply source of Na is lost.
Regarding to the addition of Na, a method for forming a film of soda-lime glass between a Mo electrode layer and a substrate has been proposed (NPL 1). However, when a film of soda-lime glass is formed as in NPL 1, production processes are increased and productivity is degraded.
Because of this, a technology, in which a sodium compound is added to a Cu—In—Ga (referred to as CIG below) precursor film to provide a Na source to the light-absorbing layer, has been proposed as disclosed in PTL 3.